Split ratio trade-off between output torque and braking torque of surface-mounted permanent-magnet machines

This study deals with the optimisation of surface-mounted permanent-magnet (SPM) motor for the vehicle door of high-speed railway transport systems, where both high-output torque under healthy conditions and low braking torque under fault conditions are required. The split ratio of rotor outer diameter to stator outer diameter is one of the most important design parameters due to its significant impact on the output torque. As found in this study, it also has significant effect on the braking torque. Therefore, the split ratio for SPM machine with consideration of braking torque is investigated. The relationships between the split ratio and both the output and braking torque are analytically established. It is found that the maximum output torque and the maximum braking torque correspond to the same split ratio. Therefore, considering the short-circuit operating condition of the door system, the split ratio related to the maximum output torque no longer satisfies its requirements. It is demonstrated that the split ratio can reduce the braking torque under a short-circuit fault. The analytical results are verified by the finite-element analyses. The experiments on the prototype 12-slot/10-pole SPM machine validate the analyses.